The present invention relates to a method and apparatus for descrambling a television signal which has been scrambled so as to disable unauthorized viewers from watching programs in a CATV system.
In general CATV systems, programs produced in a center and those aired are transmitted to terminal equipment through cables. Part of programs produced in the center are made chargeable. When a viewer at a terminal watches such a chargeable program, a predetermined fee is charged to him.
Thus, to disable unauthorized viewers from watching the chargeable programs, video signals of the chargeable programs are intentionally disorganized, i.e., scrambled at the center so that unauthorized television receivers cannot correctly receive them. On the other hand, the authorized viewers have a descrambling apparatus for restoring scrambled video signals to normal ones.
Thus far, various types of the scrambling and descrambling systems have been proposed. For example, in the so-called GSS (Gated Sync Suppression) system, only horizontal and vertical synchronizing signal sections of a television signal are suppressed by several dBs to produce a scrambled signal so that usual television receivers cannot establish the horizontal and vertical synchronization. On the other hand, in the authorized terminals, the horizontal and vertical synchronizing signal sections of the video signal are restored to the original levels.
FIGS. 7(a)-7(d) show a scrambling process in the aforementioned GSS system.
FIG. 7(a) shows a normal television signal of a base band. Character H in the figure represents a horizontal synchronizing signal. FIG. 7(b) shows the television signal modulated to an intermediate frequency video signal IF.sub.v. At the transmission side, namely the center, the horizontal synchronizing signal section and the vertical blanking interval (VBI) section of the intermediate frequency video signal are suppressed by a particular amount, such as 6 dB or 10 dB, as shown in FIG. 7(c).
This suppression is conducted during a period t.sub.1 of e.g., 12 .mu.sec including the horizontal synchronizing signal period H and during the VBI. At the center, furthermore, a key signal representing the suppression timing is superimposed on an FM audio signal IF.sub.a (FIG. 7(d)). The resultant audio signal and the intermediate frequency video signal IF.sub.v (FIG. 7(c)) are modulated to an RF signal and transmitted to the terminal equipment.
The terminal equipment extracts the audio signal IF.sub.a from the received RF signal and AM-detects the key signal from the audio signal. After that, the terminal equipment restores, on the basis of timing of the detected key signal, the horizontal and vertical synchronizing signal sections of the video signal, which have been digitally suppressed, to the normal levels by extending them by 6 dB or 10 dB corresponding to the suppression rate at the transmission side. The period of the horizontal synchronizing signal section during which the video signal is to be extended by the terminal equipment is set to 10 .mu.sec for example, which is slightly narrower than the suppression period. This is to stably operate the descrambling circuit and to prevent an edge portion of the picture section from being eliminated.
On the other hand, with respect to the VBI, when the terminal equipment detects the last horizontal synchronizing signal H preceding the VBI by, for example, a free-running timing generator which is initiated by the key signal K (i.e., when the terminal equipment detects a timing pulse which is generated, e.g., 96 .mu.sec after the arrival timing of the last key signal K preceding the VBI), it recognizes the VBI and performs the extension operation in the VBI.
Furthermore, to more effectively disable unauthorized viewers from watching (pirating) chargeable programs, a scrambling means at the center randomly changes the temporal relationship between the key signal superimposed on the audio signal and the horizontal synchronizing signal section of the television signal. For example, six kinds of the temporal relationship are used; an extension pulse occurs 5.times.N .mu.sec (N =0-5) after the key signal goes down.
The information N representing the temporal relationship (time difference) between the key signal and the horizontal synchronizing signal section of the television signal may be superimposed on the audio signal as in-band data, together with the key signal corresponding to the synchronizing signal section of the subsequent field following the VBI. This information is named "timing mode data" and the value N can be set for each field.
FIGS. 5(A)-5(G) show the operation as described above. That is, the figure describes the VBI of the video signal and the field portion just preceding it.
FIG. 5(A) shows synchronizing signals in a video signal; FIG. 5(B) shows a sync suppression pulse on the scrambling side; FIG. 5(C) shows a key signal superimposed on an audio signal; and FIG. 5(D) shows timing of extension pulses which occur on the descrambling side in synchronization with the fall (leading edge) of the key signal.
As was described above, the descrambling side obtains the key signal shown in FIG. 5(C) and generates a horizontal sync extension signal EH having a duration of 10 .mu.sec 5.times.N .mu.sec after the fall of the key signal of FIG. 5(C). Furthermore, the descrambling side generates a VBI extension signal EV 96 .mu.sec after the fall of the key signal.
Thus, as shown in FIG. 5(E), there occurs in the horizontal synchronizing section non-restored pulses (so-called H-rabbit ear) having a duration of about 1 .mu.sec before and after the horizontal synchronizing signal H. On the other hand, there occurs in the VBI section a non-restored pulse of a long duration (so-called VBI-rabbit ear). It is noted that the width of the VBI-rabbit ear varies depending on the relationship between the key signal and the suppression timing of the horizontal synchronizing signal section, which is represented by 5.times.N .mu.sec.
The operation at the VBI termination is shown in FIGS. 6(A) through 6(G) which show the signals just before and after the VBI is terminated.
FIG. 6(A) shows synchronizing signals in a video signal; FIG. 6(B) shows sync suppression pulses on the scrambling side; FIG. 6(C) shows key signals superimposed on an audio signal; and FIG. 6(D) shows extension pulses generated on the descrambling side in synchronization with the fall (leading edge) of the key signal.
In the conventional descrambling apparatus, a VBI extension pulse is turned off when the key signal corresponding to the horizontal synchronizing signal of, for example, the 23rd line goes down, as shown in FIG. 6(D). However, at the scrambling side, the sync suppression pulse is turned off at a position after the horizontal synchronizing signal of the 23rd line.
Thus, as shown in FIG. 6(E), a VBI-rabbit ear whose width is 5.times.N .mu.sec (N=0-5) occurs, and 10 .mu.sec after that an H-rabbit ear occurs.
The two types of the VBI-rabbit ear which occur before (FIGS. 5(A)-5(G)) or after (FIGS. 6(A)-6(G)) of the VBI affect the reception and reproduction performance of a television receiver. In particular, in television receivers whose separation characteristics between the video intermediate frequency signal and the audio intermediate frequency signal is insufficient, "buzz" noise occurs in the demodulated sound at every 1/60 sec.
In addition, since a white level pulse is present before and after the VBI, the entire screen of a television receiver may become whitish due to the reflection in a CRT of electron beams corresponding to the white level pulse. (The scanning position of the white pulse itself is located outside of the screen.)